This invention relates to coupled cavity travelling wave tubes and in particular, though not exclusively, to millimeter coupled cavity travelling wave tubes, that is to say travelling wave tubes for operation at a frequency in the region of 35 GHz and up to and beyond 95 GHz.
A typical coupled cavity travelling wave tube as at present known is illustrated in FIGS. 1 and 2 of the accompanying drawing of which,
FIG. 1 is a longitudinal section through part of the slow wave structure of the tube, and
FIG. 2 shows the slow wave structure in cross-section along the line A..A of FIG. 1.
Referring to FIGS. 1 and 2, the slow wave structure consists of a series of cylindrical cavities 1, of constant height. In each of the walls 2 separating one of the cavities 1 from another is an arcuate coupling slot 3, 4 of which the slots 3 in alternate ones of the walls 2 are staggered relative to the slots 4 in the remaining ones of the walls 2. Coupling slots 3, 4 provide for radio frequency (R.F.) coupling between the cavities 1.
In the centre of each of the walls 2 is provided an axially aligned drift tube or ferrule 5 through which, in operation, the electron beam passes down the length of the slow wave structure from an electron gun (not shown) to a collector (not shown).
The dimensions and positions of the drift tubes 5 influence both bandwidth and efficiency.
The method of construction normally employed to construct the slow wave structure illustrated in FIGS. 1 and 2 is as follows.
The slow wave structure is made up of sections each of which comprises a wall 2, a coupling slot 3 or 4, a drift tube 5 and a short length of the cylindrical wall 6 of the slow wave structure. Each section is blanked out and then machined to achieve the final dimensions.
The complete structure is then built up by stacking one section upon the next together with brazing wire or foil suitably placed in between.
The whole assembly is then jigged for alignment and furnace brazed.
Whilst such methods of assembly are very satisfactory for coupled cavity travelling wave tubes for operation below millimeter frequencies, it is believed that if applied to millimeter coupled cavity travelling wave tubes difficulties would be experienced due to the extremely small tolerances which would be permitted in the dimensioning and assembly of the slow wave structure of such tubes. It will be appreciated that in a millimeter coupled cavity travelling wave tube, the overall dimensions of each cavity in the slow wave structure are very small and in each tube typically three slow wave structures may be required each containing up to thirty cavities. It is estimated that for consistent performance, particularly for a tube operating in the region of 95 GHz, dimensional and assembly tolerances of the order of 0.0001" (one ten thousandth of an inch or in other words 0.00254 mm) are required.
Tolerances of this order cannot be achieved by conventional machines under normal workshop conditions. To carry out the machining of the individual sections as hereinbefore described in the quantities which would be required for normal production, would require high precision lathes and control equipment, of the type used for the diamond turning of optical components, operated in a closely controlled environment. For production purposes, providing such machining is considered to be impracticable and even if it were not the yield of good sections would likely be low.
Even with individual sections machined within tolerance, building up the complete structure by stacking the individual sections militates against the maintenance of dimensional tolerances during brazing.
Associated with the problems of manufacture outlined above are difficulties relating to inspection, measurement and handling.